1. Technical Field
The present invention relates to a projection lens system of a projector that enlarges and projects an image displayed on a light valve, such as a liquid crystal panel or a DMD, onto a screen.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 2002-357769 discloses the use of a pair of plastic aspherical lenses in a projection lens device used in a projector. In this publication, the aspherical lens of the front side has a negative power and the aspherical lens of the rear side has a positive power, so that when these lenses made of plastic deform due to heat and the refractive powers thereof change, the effects of such deformation are cancelled out by one lens having a positive power and the other lens having a negative power, which suppresses the drop in aberration-correcting performance.
Aspherical lenses are often introduced into lens systems to correct aberration at a high level using a small number of lenses. Aspherical lenses can be realized by glass lenses, but due to cost and the ease of machining, plastic lenses are often used. However, since plastic lenses have a high coefficient of thermal expansion, changes in form due to heat conversely cause a drop in aberration performance. In particular, in a rear projector such as that shown in FIG. 1, a projection lens system is disposed in a completely closed environment and so is more susceptible to the effects of heat compared to a conventional projector system in which the screen and projector are separate and at least part of the lens system is exposed.
The rear projector 1 shown in FIG. 1 includes, inside a housing 2, a light source 3, a light modulator (light valve) 4 that modulates light from the light source 3 according to an image signal to form an image, a projection lens system 5 that projects projection light 8 from the light valve 4 onto a screen 9 from the rear surface side, and mirrors 6 and 7 that reflect and guide the projection light 8 to the screen 9. Although there are cases where a CRT, in which the light source 3 and the light valve 4 are integrated, is used, in recent years liquid crystal panels have often been utilized as the light valve 4, and there are also cases where a DMD panel, formed of micromirror elements, is utilized. In the case of a DMD panel, the light valve 4 is a reflective-type, so that the positional relationship with respect to the light source 3 differs to that shown in FIG. 1. However, the input (incident) side of the projection lens system 5 is telecentric regardless of whether a liquid crystal panel or a DMD panel is used as the light valve 4. Accordingly, a projection lens system 5 that enlarges and projects the modulated light (projection light) 8 and is telecentric on the input (incident) side is required.
In view of the precision of aspherical surfaces, it is easier to form a suitable surface for correcting aberration when the area to be aspherized is large. However, increasing the lens diameter of an aspherical lens has the demerits of a large increase in cost for a lens made of glass and of increased effects due to thermal deformation for a lens made of plastic. In a projection lens system, the lens closest to the screen has the largest diameter, but as disclosed in Japanese Laid-Open Patent Publication No. 2002-357769, in many cases it is not the lens closest to the screen but the next lens or a lens located at an intermediate position that is aspherized.
In a lens system with an extremely simple construction where the lens system is constructed of only a few lenses, the number of surfaces at which an aspherical surface can be introduced is limited. Therefore, there are cases where one or both surfaces of the lens closest to the screen that has the largest diameter is/are aspherized. With a lens system of such a simple construction, for a conventional projector that is separated from the screen, the lens closest to the screen is subject to the weakest thermal effects and so hardly deforms due to heat. Also, since a simple system is not expected to achieve a particularly high aberration correcting performance, the aspherical lens is only provided with a corrective performance such that thermal deformation of the aspherical lens will not have an especially large effect on the aberration performance of the entire lens system.
However, in the rear projectors that are currently being introduced, due to conditions relating to the placement of the lens system, the effect of heat is much greater than in a conventional projector. Image quality that is at least equal to that achieved by current CRTs and liquid crystal TVs is also required. In addition, low cost remains a constant requirement. In a method, such as that described in Japanese Laid-Open Patent Publication No. 2002-357769, that overcomes the effects of thermal deformation of aspherical lenses by increasing the number of aspherical lenses, the cost required to design and manufacture the aspherical lenses increases, and it is necessary to improve the design precision and raise the manufacturing yield so as to avoid a drop in the aberration performance due to thermal deformation of the additional aspherical lenses, which also results in high costs.
For this reason, it is an object of the present invention to provide a projection lens system that has high aberration-correcting performance, that can suppress deterioration in the aberration-correcting performance due to heat, and that can be supplied at low cost.